Arch Oboler's Lights Out often demonstrated how one could be economical with actors, yet effective at building terror. "Spider," from June 8, 1938, casts just two men, but their dialogue carries all the horror required. In this tale, these men have ventured into the jungle to capture butterflies for money when they find a spider "as big as a dog." This could be a valuable discovery, they assume; however, while it's possible to capture the spider, holding it is another matter.
You can download a copy of this radio show from archive.org here.