I get asked this all the time at meetups, because 'The Lovers' is a title that keeps cropping up for different films through the decades.
If you mean the 2017 indie film 'The Lovers' (the one I caught at a tiny theater and loved for its awkward, human comedy), the main stars are Debra Winger and Tracy Letts as a married couple whose long relationship has become strained and flirtatious in very adult, messy ways. Aidan Turner also appears as a younger man who becomes involved and shakes things up—he's basically the outside spark that highlights the couple's boredom and desire. The movie leans into their chemistry and the moral ambiguities of midlife romance.
If, instead, you mean the classic 1958 film titled 'The Lovers' ('Les Amants' by Louis Malle), that one famously stars Jeanne Moreau (the woman at the emotional center of a scandalous affair) opposite the male lead who becomes her lover; it's a different mood entirely—more tragic and art-house. If you had a specific year or actor in mind, tell me which one and I’ll dig into the exact character names for you.