A full-length trailer for Daniel Radcliffe's first romantic comedy What If has been released online.

Undeterred, he attempts to win her over with help from his mate, played by Girls star Adam Driver.

[related]Speaking to Yahoo! movies, Radcliffe revealed that he knew he had “chemistry” with co-star Kazan straight away.

“I think it was probably the first scene in the diner, when we were just throwing stuff out there and trying to make each other laugh," he explained.

“It was obvious from then that we were going to be able to work well together and play off each other well.”

Directed by Michael Dowse, the movie also features Rafe Spall, Megan Park and Jemima Rooper.

What If hits cinemas in the US on 1 August and 22 August in the UK.