As I know, verbs are followed by adverbs. So, which is true? "She stands there completely expressionlessly" Or "she stands there completely expressionless" Though I think the second one is likely to be true, I can't find out a satisfactory explanation. Thanks in advance!
expressionless is modifing she, not stands, so it should be an adjective, not an adverb. It's equivalent to
She's standing there and is completely expressionless.
An example where you could use an adverb is:
She's standing there completely effortlessly.
The use of the adverb completely is irrelevant. An adverb can modify either an adjective or adverb. Both sentences would be the same with or without it.
Adverbs ordinarily precede the adjectives or adverbs they modify, so that is why in your examples "completely" precedes the adjective "expressionless" or the adverb "expressionlessly", which it modifies. "Completely" can also modify a verb, in which case it can precede the verb it modifies: "They completely missed my point." However, it can also follow the verb phrase: "They missed my point completely." This particular type of adverb does not occur at the beginning of a sentence: *"Completely they missed my point." Nor can it occur before an auxiliary verb: *"They completely have missed my point." But many other adverbs can occur in the two latter positions.
The position of adverbs depends on what the adverbs modify, but unfortunately not in any simple way.
Both "expressionless" and "expressionlessly" are possible in your example, and "completely" can precede and modify either one, since adverbs can modify both adjectives and other adverbs.