Lily Collins had to train three months before shooting scenes for 'The Mortal Instruments: City of Bones'.

The 24-year-old actress admits she's generally an active person, but she had to work out every day for 90 days prior to the film's filming schedule kicking off and every day from then on because she feared that she wouldn't be able to keep up with her co-stars in heart-pumping scenes.

She said: ''I trained for three months before filming in Los Angeles, then every day before during or after work with the stunt department, as well as our physical fitness trainer. I've always been an active person, and I really enjoy getting down and bruised, I guess.

''With this one I had to hang with all the guys, and keep up. In heels, and a very short dress.''

However, the brunette beauty - who plays Clarissa ''Clary'' Fray in the action adventure fantasy film - insists despite her hard training sessions, she still struggled with filming some scenes because her alter-ego's costume weighed her down, but she's glad she never injured herself.

She explained: ''I guess the hardest part for me, physically, was that I didn't train in the outfits except for the day before. Then I'm thrust into it with these heels and this dress.

''As Lily it was very awkward, but I could just pawn it off on Clary. I never twisted an ankle and I barely tripped, I just bruised a lot.''

'The Mortal Instruments: City of Bones' is in cinemas now.