This is a fine old melodrama. It's really much too long and complicated to be easily adapted to a play (although Collins did so, apparently, to protect his rights). But there is decidedly something of the Victorian theatre in the easily contrasted main male characters, the wild coincidences, the spectacularly atmospheric conclusion in a (sort of) madhouse, the foreboding dream visions that work their way into reality, and, of course, our wonderfully scheming villainess (or anti-heroine), Miss Gwilt. Then you have a gaggle of minor characters, none any better than they should be, who would provide meaty parts for the character actors.
Collins plays with his characters, lining them up in pairs for contrast, in a way that was probably second nature to him, since (as I noted) he was doing it in his very first novel, Antonina. Here, however, the strategy is made even more obvious by the laughably improbable coincidence of two pairs of characters in two generations, all named Allan Armadale. Fortunately, they troop on to our stage no more than two at a time, and in the second generation, which takes the meat of the story, the coincidence of names, though a plot point, doesn't bother us at all, because one of the Armadales calls himself Midwinter throughout (except on one crucial document - I say no more).
The length of the novel notwithstanding, Collins indulges in far less discursive padding here than in his first novel. He also varies his narrative style, going to letters and diaries as we penetrate closer and closer to understanding the feelings and motives of the woman who is going to kill one of the Armadales - ah, but which one?
There's less depth to the characters than Dickens provides, but I must admit I enjoyed the gradual unspooling of this suspenseful tale. (P.S. a little searching turns up a 2008 play adaptation by American playwright Jeffrey Hatcher, and also a 3-part radio adaptation the following year on BBC Radio 4, by Robin Brooks. Since Collins actually gives Lydia Gwilt a somewhat plausible psychological cause for her villainy, in early marital abuse, I wonder if that aspect was played up in the 21st-century adaptations?)
Anyway, if you're in the mood for meaty Victorian fiction and you've exhausted your Dickens shelf, Armadale is good fun.