The full sentence:
藤木がハッとしたように言った。
「thing Fujiki said」
Fujiki said with surprise ...
How does this differ from just saying
藤木がハッと言った。
Is したように necessary, and if so, why, and what extra is it adding?
Note, I'm happy with how the するように construct works normally. I'm just asking about whether it might be redundant in this sentence since ハッと is already an adverb.
「藤木がハッと言った。」
If this sentence meant anything, it would be none other than:
"Fujiki said 「ハッ」."
In other words, to native speakers,
「藤木がハッと言った。」＝「藤木が『ハッ』と言った。」
「ハッと」 surely functions adverbially, but it can only modify selected verbs such as する、[驚]{おどろ}く、[気]{き}づく, etc. 「[言]{い}う」 is not one of those verbs.
I assure you that there is no redundancy whatsoever in the sentence:
「藤木がハッとしたように言った。」
Perhaps, you would need to treat 「ハッとする」 like one word here.
藤木がハッと言った is translated as Fujiki said "ハッ".
ハッとする means "get startled" and ように is a 連用形 of verbal auxiliary ようだ which means "comparison", so ハッとしたように is translated as "like one is startled".
藤木がハッとしたように言った is translated as "Fujiki said like he was startled"
