A mockumentary extending a series about a hapless interviewer roasting celebrities, “Between Two Ferns: The Movie” doesn’t need to rope in a decent plot (it doesn’t) nor to exhibit any acting ability from star/creator Zach Galifianakis (it doesn’t), but it does need to generate regular belly laughs, and in this regard, the film almost works but fails. The film intelligently varies the settings by sending our dumb griller on a dumb road show, and the “star’s” back-up crew is solid (especially the gormless assistant portrayed well by Lauren Lapkus), but Scott Aukerman’s script and direction drift. Will Ferrell, as the manic producer, hams things up for no laughs at all. As for the star turns, scenes with Bruce Willis and Keanu Reeve are plain dull, and only deadpan, hilarious segments featuring Benedict Cumberbatch and Matthew McConaughey save “Between Two Ferns” from major torpor.