When you play Slithering Marauder and its' ability lets you play an action out of your crypt, the spell will resolve before choosing targets. For example, if you play Burn, you don't choose your target until the spell itself resolves (in addition to Slithering Marauder's ability.) Against the AI, this may not matter, but a player would notice.

Visually, you can see the action got to the void (as part of its resolution) before you select your target.