Entirely improvised and shot with multiple vid cameras in a single day, “Invisible” is an ambitious experiment that, alas, is more intriguing as a concept than as a completed pic. Multi-hyphenate Adam Watstein’s bare-bones indie drama begins as a morosely melancholy study of a thirtysomething couple on the verge of divorce, then devolves into an unpleasant thriller about their confrontation with psychos. Imagine a slasher pic directed by Henry Jaglom, and you’ll have some idea what to expect. After limited theatrical play, “Invisible” should live up to its title.The extended set-up forces aud to spend an unonscionable amount of time in close quarters with Joe (James Tupper, of TV’s “Men in Trees”) and Jane (Kit Pongetti), estranged marrieds who barely can speak to each other in a civil tone. During a long drive to their secluded cabin, they bring out the worst in each other. When they finally commune with nature, their hostility begins to fade. But their reconciliation is interrupted by two doltish wack jobs (David Mogentale, Joe Mellis), who terrorize the couple while insisting Jane is their long-lost mom. Nothing good comes of this.