Ashton Kutcher struggled to keep up with Natalie

Ashton Kutcher found working with Natalie Portman "a blessing and a curse".

The actor stars in romantic comedy 'No Strings Attached' with the brunette beauty and as he considers her to be one of the best actresses around, he admitted he had to make sure he was fully prepared every day to keep up with her on set.

He explained: "She's probably one of the best actresses of my generation. I believe she is. It's a blessing and a curse. One, you know everything you get across from you is going to be real and honest and reactive and beautiful. At the same time when the light shines in a dark room, all the dust shows. It was a challenge for me to really play this role and be as honest as she is in every scene."

Ashton also revealed doesn't always find it easy to have chemistry with co-stars but said Natalie is so naturally charismatic she made it easy.

He told BANG Showbiz: "It's easier when you like the person. And she's very likeable. I don't know if it's something you can plan or if it's something you can manufacture.

"I generally find that you can have really good chemistry with someone that you can argue well with, and I think both Natalie and I really appreciate being right. I think that makes for good chemistry. But I think that Natalie could have chemistry with a cup if she wanted to."