Sean Hunt said he was grateful for the opportunity as he helped put Sussex in command at Hove.
The seamer took a career-best 5-48 from 12 overs to help bowl out Somerset for 201.
Sussex, with Tom Haines making a century, were 246-1 at close, a lead of 339.
Hunt, called in with Ollie Robinson hit by a foot injury, starred on day two of the Rothesay County Championship division one match.
He said: “I’m very grateful to get the opportunity to play. I wanted to play last week but when you’ve got Jayden [Seales] and Ollie in the team, it’s hard.
“But, when I get the opportunity, it’s important to take it. I was really excited.
“It’s not an easy challenge because they bat all the way down to eleven.
“The pitch has lost a touch a pace and got better. Hopefully it will break up as the game progresses.”
Hunt believes he is better-placed to make big contributions for his county.
He added: “I worked on a few things in the winter, like my head position and my run-up and I feel I’m better for it.”
An opening partnership of 184 between Haines and Daniel Hughes placed Sussex in a commanding position.
By the time Jack Leach finally broke the first wicket stand, in the 50th over, the Sussex lead was 277. And by the close they had extended their advantage to 339 thanks to a 13th first-class century from Haines, his first in the top division.
Sussex will be looking for a lead in the region of 500 runs on the third day. It will take a big total to bat Somerset out of the game. Somerset will remember how, last June, they chased down 410 in the fourth innings to beat Warwickshire by five wickets with four overs to spare.
They will know how the pitches at Hove can flatten out in the course of a match.
Sussex, for their part, are probably still haunted by the memory of Louis Kimber’s astonishing 243 here last season, which almost pulled off a far-fetched victory for Leicestershire, who were set 464 in the fourth innings and lost by just 18 runs.
A century looked there for the taking for the Australian left-hander Hughes. But on 91, attempting to cut a wide delivery through backward-point, he was caught behind. He had faced 151 balls and hit 16 fours.
Haines, though, would not be denied. He moved to 99 by hitting Archie Vaughan for a straight six and then hit Leach back over his head to reach three figures. He had faced 182 balls and hit 12 fours and two sixes. And at stumps he was still unbeaten on 117.
The morning session had three distinct phases. Sussex took two quick wickets, to maintain their dominance of the previous evening. James Rew then led a considerable Somerset counter attack before the last three wickets fell for just 12 runs runs.
Somerset resumed on 62 for five, still 232 runs behind, and their chances of mounting a meaningful recovery were seriously compromised when they slid to 79 for seven.
In the third over of the day Hunt found the inside edge of Tom Lammonby bat and wicketkeeper John Simpson took a sharp catch moving sharply to his left. The left-hander had not added to his overnight score of 24. Two overs later the same combination accounted for Kasey Aldridge, this time Simpson completing a jugging catch.
But from that unpromising position Rew and his captain Lewis Gregory took the attack to the Sussex bowlers and added 110 for the eighth wicket in 20 overs. When Rew clipped Danny Lamb through mid-on for four to reach 49 it took Somerset to 145, and they had avoided the chance of being asked to follow on.
Rew then edged Seales for his eighth four – one of his few false strokes – to reach his fifty from 61 deliveries. It was the first half-century of the match. Rew edged Seales for another boundary, this time to raise the 100 partnership.
The partnership was progressing so well that it was a surprise when Gregory, who had hit four fours in his 36, lifted Hunt to Seales at mid-on. In Hunt’s next over Migael Pretorius edged behind and the bowler ended the innings when he had Leach also caught by Simpson, first ball.
Somerset had lost their last three wickets in 15 balls, leaving Rew unbeaten on an 83-ball 80. Left-armer Hunt’s figures of five for 48 from 12 overs were the best of his career.
