Clooney was unproven as a film star though, a bit like Bruce Willis at the time of Die Hard. Kilmer had been around for some time but mostly as a supporting or team player (Top Gun, the aforesaid Tombstone) and few of his films where he headlined were big hits (Willow, Thunderheart, Kill Me Again, The Doors). He wasn’t a Tom Cruise or Mel Gibson, for example.Affleck, Kilmer (Top Gun, Willow, the Doors and Tombstone) and Clooney (mostly for ER but that alone made him a household name) were all big names at the time they played Batman.
ScarJo was arguably a big name by the time she played Black Widow, having appeared in a couple of Woody Allen movies (including Match Point), the Island, Lost in Translation and the Prestige. Probably bigger than Ruffalo or Norton IMO.
Good call on ScarJo - indeed, Gwyneth Paltrow was also probably a bigger name at the time of IM than many of the male MCU stars.