The most effective function of a subplot is to show how the protagonist is transformed by love.
After I had the stiff drink I needed when I discovered that a third of my romantic comedy screenwriting students had no idea that 2011's Arthur is a remake, I took another look at the original. This nearly-a-classic, written by the sorely missed Steve Gordon, holds up pretty well if you can overlook how the whole gestalt of the thing (i.e. alcoholic billionaire elitist as lovable jokester hero) has badly dated (for what went wrong with the remake, go here).
The original's laugh-per-lines percentage, however, is pretty formidable, especially by today's paltry standards. And one thing that the first Arthur (1981) did really, really well - earning Sir John Gielgud an Oscar, for his supporting role as Hobson the butler - was to provide a secondary story line that gave the whole movie a substantive spine.
Arthur is an overgrown child, and Hobson is his surrogate father, who both indulges and actively disapproves of his boy-man employer. Hobson is given some of the movie's best lines at Arthur's expense, one of which has since become part of the cultural lexicon (Arthur: I'm going to take a bath. Hobson: I'll alert the media).
As the story unfolds, Arthur falls in love with Linda, a "nobody from Queens" played by Liza Minelli, despite his being engaged to marry wealthy Susan (a memorably scary Jill Eikenberry). As is true of all good rom-coms, falling in love causes our hero to shift from his status quo and grow; the genre's ethos demands that love be a positive agent of transformative change. But how does one show such a change?
Writer-director Gordon shrewdly manages this through the arc of Arthur's relationship with Hobson. Early in Act 2, Arthur learns that Hobson is ill. Visiting the bedridden butler, Arthur becomes furious when Hobson insults Linda. He yells at him and storms out, slamming the door - only to promptly re-enter, apologetic. Arthur: Hobson, I raised my voice to you. I've never done that before. Hobson: That's quite alright. You know... you may be growing up. And I'm sorry for what I said about Linda. Arthur: You want anything? Hobson: I want to be younger. Arthur: I'm sorry, it's your job to be older.
Arthur's arc is laid out for us quite succinctly. He is indeed growing up (as his love for Linda forces him to make hard choices and suffer the consequences) and the subsequent development - Hobson is in fact terminally ill - shows us Arthur accepting his job to be older: child becomes father to the man as Arthur chucks his beloved bottle and takes care of Hobson in the hospital, until his eventual demise.
Hobson's absence leaves Arthur standing on his own two feet, a sober adult. Finally man enough to stand up to his family, he rejects their woman of choice, choosing the nobody from Queens and losing his millions (at least for a Dark Moment's worth: this is a comedy, after all). Without the masterful Hobson subplot, the wake-up call from death that underscores Arthur's new love-inspired joie de vivre, we'd have no way of tracking his arc, of really seeing how Arthur is transformed by love.
So many romantic comedies get this wrong! I read spec scripts all the time where a cursory subplot is dragged into play, simply to provide some exterior story line that'll pad out the picture, to "open things up" beyond the central romance, and raise the stakes in it by ginning up some work-related threat. No, silly rom-com writers - your subplot exists to illustrate how your hero or heroine is transformed by love. If it's not showing the audience proof of the protagonist's shift in character... it has no reason to live.
Yes, some rom-com subplots go about this job by comparing and contrasting what the protagonist is going through (see the Bruce-and-Marie coupling in When Harry Met Sally...), but the effect is the same. And when the workplace is involved, whatever's going on there should be showing us that character-driven transformation; witness how Tom in (500) Days of Summer comes into his own as an architect by the movie's end (just as Summer learns that she's ready, ironically, to be with someone else for good).
What's the lesson-learned for your primary protagonist? What's the specific effect of love walking into his or her life? Your subplot ought to be an active acting-out of that transformative arc, and if you're looking for a worthy role model on this front, it's well worth looking into Arthur. Not the Russell Brand one. Did I mention that there's another?