“Staged,” a six-part season of brief episodes about actors in lockdown, could only have been conceived in lockdown (by Phil Glynn and Simon Evans, who also writes and directs) and only with superb improvisers like Martin Sheen and David Tennant. That Tennant and Sheen are themselves friends helps, for the pair riff off each other about mock competitiveness and insecurities. The storyline is gossamer thin: a budding producer/director tries to get the two stars to rehearse an old Italian play during lockdown, and various other stars (Samuel Jackson, Judy Dench) butt in. Cleverly, the scenes play out as Zoom sessions interposed by footage of pandemic emptiness. The interplay between Tennant and Sheen is magical, often profane, and razor sharp. All in all, Staged is a Coronavirus boon, one to be spaced out and savored.