Jessica Hecht grinds her teeth. A lot. You wouldn't guess it as the smiling actress sips a mochaccino in a laid-back L.A. café. But this is a rare moment of relaxation for Hecht, 29, who is featured on two hit NBC sitcoms this season. On Friends, she's Susan, the acerbic lover of Ross' ex-wife; on The Single Guy, she's Janeane, the motherly best friend of the title bachelor. Both great gigs, but Hecht's molars have been feeling the pressure: ''My husband and I both have these night [mouth] guards, like an old couple going to bed.''
The grinding was even worse last season, when she worried about how gay viewers would respond to her portrayal of Susan. (Hecht had originally read for the role of Monica which went to Courteney Cox. ''Can you possibly think of two people less likely to be cast in the same part?'') ''The producers were extremely clear that no stereotypes should be indicated,'' Hecht says. Her fears were allayed when gay groups, such as GLAAD, cited Hecht and Jane Sibbett for their positive portrayal of a lesbian couple.
Indeed, Hecht did such a good job as the astringent Susan that NBC execs doubted she could also play Single Guy's mushy Janeane. ''My Susan persona had been burnt into their mind,'' says Hecht. ''I just felt like, please spend another five minutes with me and I promise I won't bite your head off.''
Earlier in her career, Hecht's attitude was ''I don't want to be a sellout and do TV.'' She came from an intellectual Bloomfield, Conn., family: Her mother, stepfather, and both sisters are psychotherapists; her father is a physicist. While studying drama at NYU, she supported herself as a dance-oriented personal trainer to such clients as Isabella Rossellini, James Taylor, and Ellen Barkin, with whom ''I always treaded lightly on talking about work.''
In 1991 the national tour of The Heidi Chronicles brought Hecht to L.A., where she landed a job as nanny to the three kids of Cheers' George Wendt (and met It's Pat director Adam Bernstein, whom she married last summer). Wendt convinced her of TV's merits, and she got cast in MTV's You Wrote It, You Watch It and guest shots on such shows as Seinfeld and Lois & Clark.
Friends and Single Guy have changed her attitude for good. ''All these years I did obscure plays because I was trying to enlighten somebody about the human condition,'' Hecht says. ''Now I just love entertaining people.''