A source tells the mag that the funnyman "is totally smitten. It's like the first girl he's ever been this way with." The actress is equally head over heels and "hasn't been this happy in forever."
The seriously adorable pair (you just can't help but root for them, right?) were spotted strolling in New York on Sunday, three days after they held hands while socializing at the afterparty for the New York premiere of Williams' new movie, "Take This Waltz."
At the screening, the New York Daily News asked the actress, 31, if Segel would now consider more serious roles because of her Oscar-nominated status. Her pitch-perfect response: "He's his own man."
The couple also looked "lovey-dovey" at a Broadway show last week, and Us says Jason, 32, has been doing "dad stuff ... like drawing and playing music" with 6-year-old Matilda, Michelle's daughter with the late Heath Ledger.