Alexandra Burke talks ahead of Sister Act role at Wolverhampton's Grand Theatre

The West End star was leading such an unhealthy lifestyle with erratic working hours and endless takeaways that her weight spiralled. A normally svelte 8/10 figure suddenly became a curvalicious 14.

A holiday snap from Ibiza left her stunned. So she hit the gym, ditched the pizzas and started eating sashimi for lunch. "I was thinking to myself, 'Why I am I buying size 14 pants?'. I'm normally an 8-10 but I just got on with it.

"Then my boyfriend took a picture of me by the pool. As we got on the plane to go home, I looked at the photos and I thought 'Why do I look like this?' He told me, 'Yeah, you have put on a bit of weight, but so have I'.

"I actually got involved with SlimFast simply because I had a situation on stage when I was in The Bodyguard and realised I had put on weight.

"The most embarrassing thing that could ever happen to an artist is that you don't fit into your clothes and your dresser has to put the biggest safety pin in your dress in order for you to continue on stage. That's exactly what happened to me."

When Alexandra returns to Wolverhampton's Grand Theatre on Monday to star in Sister Act, she'll be the hardest, toughest and most bad ass Sister Delores that the show has ever seen. She's got abs that look like they've been honed from teak and the most sensuous figure this side of Models 1.

She was offered the role of Deloris in Sister Act after dazzling audiences and critics with her performances in The Bodyguard. She took over the role from Wolverhampton's Beverley Knight, making it her own.

And now the former X Factor winner is back on the road in a new production of Sister Act that's been directed and choreographed by Strictly Come Dancing judge Craig Revel Horwood.

"I got offered the role after Jamie Wilson, the producer, came to see me in The Bodyguard."

Remarkably, Alexandra said no. She didn't think she was right for the feel-amazing musical comedy, which is based on the smash hit movie starring Whoopi Goldberg and Maggie Smith.

She laughs: "That's right. I turned it down because I didn't think I could do comedy. I listened to the soundtrack and read the script and it's different to the movie. I say that in the sense of the music because the theatrical version is all original songs by Alan Menken. But we have our own musical supervisor, Sarah Travis, and Craig Revel horwood has directed and choreographed it so it became an opportunity I couldn't turn down."

And how has Craig been? The Strictly judge might seem like TV's Mr Nasty but in person he's a real charmer.

"He's been great. He's very strict and he takes it very seriously but that's because he always wants people to do the best."

Alexandra is best known for her success on The X Factor. She is one of the most successful winners of the show, having conquered her competitors in the fifth series, in 2008. After winning, her single, Hallelujah, sold more than 100,000 copies in a day and became the Christmas number one as it registered more than a million sales.

Her debut album debuted at number one and earned her three Brit nominations.

In winning The X Factor, Alexandra had demonstrated her commitment to being a pop star. In fact, she'd auditioned for the show in 2005 and made it to the final seven in Louis Walsh's 16-24 category before being rejected on the basis that she was too young.

Prior to that, she'd taken dance classes and sought theatrical roles.

"I've been acting since I was 15 and I only stopped to pursue my singing career. But Sister Act was a new challenge for me. I don't find myself funny on a day to day basis. I'm not a comedian, I'm just me. So to have that timing and stuff sounded a bit daunting, but it turns out that I'm okay at it. Craig hasn't fired me, so I must be doing something right."

Performing is in Alexandra's blood. She's following in the footsteps of her mother, Melissa Bell, who was a soul and r'n'b singer with Soul II Soul. Melissa also worked with Liza Minnelli, Whitney Houston and Stevie Wonder and presently fronts the group Soul Explosion.

Her understanding has helped Burke as she's settled into a career as an established performer.

"I don't think it ever gets easier but you get comfortable in the role and enjoy it more than on opening night. I don't want to be complacent. I'm always the one that's late to the stage just before the curtain goes up. By beginners call I'm not even in my outfit. I bang it on, say a prayer and walk straight on stage. I don't like to wait around and have a chit chat. I want to get on stage and go. Everyone that works with me knows that I want to get straight to it.

"So before a show, I'm not in my dressing room reading a book. When I'm in the theatre I'm busy with interviews or things with the company manager. It's quite hectic when I'm at work. It's never chilled for me. I've never had a relaxed time."

Sister Act tells the hilarious story of Deloris Van Cartier, a disco diva whose life takes a surprising turn when she witnesses a murder. Under protective custody she is hidden in the one place she won't be found – a convent!

Disguised as a nun and under the suspicious watch of Mother Superior, Deloris helps her fellow sisters find their voices as she unexpectedly rediscovers her own. The show was made popular by Sister Act in the 1992 film, which grossed US$231 million. It also starred Kathy Najimy, Wendy Makkena, Mary Wickes, and Harvey Keitel in a sparkling tribute to the universal power of friendship, sisterhood and music.

"The thing that's new is the music. And I find the songs quite spectacular because they hold the story together. The story is the same. It's like the Sister Act I story with the energy of Sister Act II. We don't stray from the story. But it's a lot of fun.

"It's a challenge for me vocally and in terms of my acting, but that's what I love about it.

"Performing somebody else's songs and trying to do them justice isn't easy. Added to that is the acting side of everything.

"It still makes me nervous delivering six, sometimes seven, shows a week and wanting the audience to enjoy it. People walk away from the show and actually find me funny – that is the best thing in life. I didn't think that's what I'd be able to do. So I'm really pleased. I just hope I can keep it up.

"Deloris is crazy, which I guess I am too but I'm not as stubborn.

"What's great about Deloris is, she doesn't come across very well at the beginning but later on she realises there's more to life than fame. When she realises that, she finds love.

"You really have to let everything go when you're both on the stage and in the audience. You have to be silly and goofy."

Sister Act follows on from The Bodyguard, which was an enormous success. The 2012 musical was based on the 1992 film and the opening cast included Grammy-winner Heather Headley before Beverley Knight took over her role as Rachel. The show proved so popular that it was extended and Beverley eventually made way for Alexandra.

"I loved The Bodyguard. I was on that show for a very long time, just over two years from starting it in London. From doing that show to going into this, I only had five days off. I went straight in with no break. Thank God I'm a positive person. I don't know how anyone else would take it on. But I'm loving it.

"It's been amazing.

"I have been very lucky to be able to be picked to do such iconic roles. Going from playing Whitney Houston and then now playing Whoopi Goldberg, it's just a massive honour and a privilege really to have these two roles under my belt."

She's previously appeared in the West Midlands in The Bodyguard and has happy memories of local crowds. The region is popular with Alexandra and she hopes fans will rally at Wolverhampton's Grand.

"There isn't a city that I can complain about or don't enjoy. I've been very blessed to turn up to the theatres and people welcome me with open arms. There's no complaint from me. I love it. You don't know how long you have left in this industry, so I'm grateful for it.

"When I'm on the road, I miss my family and to be honest I've never liked it, even when I was doing my own tour, because I find it really hard being away from home and my family.

"But from the moment Sister Act opens to the moment it ends, you literally see a massive live band in your face. We are all acting, singing and dancing around the stage. You don't get that every day."

With so much high quality acting work, you'd be forgiven for imagining that Alexandra has switched her focus from making music to the theatre. But that's not the case. Though her music has taken a back seat in recent years and she's not released an album since 2012's Heartbreak On Hold, she's back in the studio and itching to release new work.

She hopes it will be released later this year. However, nothing's been confirmed. And having previously voiced hopes that the record would be released by now, she's reluctant to tie herself down.

"Since the last album, a lot has changed in my life, but most importantly I have changed. I'm in a great place, healthier and stronger than ever. I feel as an artist it's important for one's music to grow with them. . . Having taken time out of the musical ring, I now have the gloves firmly back on.

"I was just in the studio two weeks ago if that and I'm still there. I have a portable studio and one in my house. So I'm still recording the third album. I think it's a matter of waiting and seeing but I can't put a date on it at the moment. I'll let the fans know as soon as we decide.

"The third album isn't finished yet but it's in the final stages which I'm happy to say out loud. So I will be returning to music, hopefully soon."

Though Alexandra has been immersed in music and acting since her childhood, she recognises her debt to The X Factor. While many former contestants seek to distance themselves from TV's number one talent spotting show, Alexandra is proud of her association.

The show made her a household name and she has no regrets about featuring on it. She realises that without Simon Cowell's support and the exposure she enjoyed in front of millions of music fans, her career wouldn't have taken off. There'd have been no number one album and, therefore, no starring role in either The Bodyguard or Sister Act.

"I have very fond memories of that time and I am constantly reflecting on things that were lifetime achievements for me. For me, X Factor set the path for where I'm going now and what I'm doing today. If it wasn't for that, I wouldn't be doing this so I'm grateful. I enjoyed it. It all happened so fast, really. Everything kind of happened really quickly.

"The last eight years of my life have just gone in a flash. It's crazy how time flies when you are having fun."

Andy Richardson

Alexandra Burke is starring in Sister Act at the Wolverhampton Grand from Monday until Saturday March 4. For tickets, costing from £26, visit www.grandtheatre.co.uk or call the box office on 01902 429 212.