Remember the halcyon days of Katherine Heigl's burgeoning career as a rom-com star? In the late '00s, Heigl's face was popping up all over the place. Propelled by her prominence and success in the the instant TV hit Grey's Anatomy, Heigl's career was resuscitated. As she shifted into more adult roles and away from teen fare like Wish Upon A Star and Wuthering Heights, it felt like Knocked Up affected Heigl's career for the better. This film came along at a time where it reinforced that she could handle more dramatic or emotionally-charged material with ease while gamely throwing herself into characters that find themselves in uncomfortable or unconventional romantic situations (a hallmark of her peak-'00s work). Knocked Up signaled a new era for the actor until that era quickly fizzled out, but it was a damn good ride while it lasted.
The entirety of 2005 effectively re-branded Heigl for the public when she signed on to Grey's Anatomy as Dr. Izzie Stevens, the young doctor who was damn good at her job but could become the butt of the joke when her beauty was brought into the conversation. This fight to push past her good looks became a smaller running theme in Heigl's career as she worked to move outside the confines of a casting type (here, the beautiful blonde) and work on meatier or perhaps more challenging films.
Knocked Up arrived at the right time for Heigl. She was two years into her work on Grey's Anatomy and she was beginning to work outside the show; the magic of Shondaland had graced her with the cachet to get considerably high-profile work, like Judd Apatow's latest unconventional rom-com. In Empire's 2007 review of Knocked Up, there's a brief appraisal of Heigl's performance which sums up (if not a little reductively) how she is actively working with and against her type to deliver something new for her acting range: "If anything, Katherine Heigl is the performance of the movie - the poster girl facing the grubby business of practical parenting with a partner intent on totting up the naked breasts in Wild Things. She is obviously a stunner (the contrast with Rogen’s wobbly slacker is crucial), but she strips away the veneer of a thousand facials to find a very credible anxiety in Alison."
Seeing Heigl as Alison deal with some very modern issues, chief among them balancing a high-profile career with impending motherhood, felt like some fun, new territory for Heigl as an actor. She carried some big emotional weights in the film that let her show off that predilection for portraying nervous or high-strung or nagging women; this aspect of her characters would resurface time and time again in films like The Ugly Truth, 27 Dresses, and Killers and it wouldn't be as charming or work to as great effect as it did in Knocked Up.
Basically, Knocked Up affected Heigl's career positively by proving that she was capable of doing more that being just another pretty face on screen; it ended up backfiring against her when the peak of her career became defined by playing a bevy of specifically rom-com women who act as the straight man to their schlubby, rakish, or not-as-competent love interests. Sure, Heigl has been able to do some high-profile work, but it's ultimately hobbled her as much as it has helped her.
I'm not going to specifically blame Knocked Up for Heigl's waning career, which has only recently begun to pick up steam again with more indie and genre-specific films like Jenny's Wedding and Unforgettable. If Heigl can revive her career in a matter of years following the incredible success that came with Knocked Up, it'll be exciting to see what she chooses to do next.