Caught in the Act

Jason Segel 'Is Totally Smitten' with Michelle Williams, Source Says

06/22/2012 at 04:30 PM EDT

A few months after going public with her romance with Jason Segel, Michelle Williams "hasn't been this happy in forever," a source tells PEOPLE.

And "Jason is totally smitten," said the source about the couple, who appeared at an afterparty for the premiere of Take This Waltz at Manhattan's Sons of Essex restaurant Thursday night. "It's like the first girl he's ever been this way with."

The couple, who only spent about forty minutes at the afterparty, sat down to chat with another couple and mingled with friends briefly before leaving. They appeared in good spirits, laughing together and occasionally holding hands.

RELATED: Michelle Williams & Jason Segel Enjoy Another PDA-Filled Afterparty

Segel, 32, and Williams, 31, who have been caught in the midst of PDA before, dated several months before one of Williams's friends confirmed to PEOPLE that the two were dating.

Take This Waltz, which debuted at the Toronto International Film Festival in 2011, opens June 29.
– Alexandra Zaslow and Jennifer Garcia