The 29-year-old Glasgow Warriors scrum-half has endured a torrid time with injuries, his appearance as a replacement in last Saturday's win over Italy ending a 17-month enforced absence from the Test arena.
Cusiter, whose most recent previous cap had been during the Six Nations win over Ireland in Dublin in March 2010, said: "It was quite emotional. I was so happy to get on the pitch and to have a chance to get selected.
"Not being fit and not being up for selection would have been heartbreaking.
"I'm just so happy to get a chance and to make the final squad."
But Cusiter, who won his 53rd cap at Murrayfield on Saturday following a long battle with a fractured kneecap, is not content merely to be part of the 30-man playing party who will travel to Australia on Saturday and on to Invercargill for the September 10 Pool B opener with Romania.
He added: "There's a bit of time now to keep working hard and put myself in a good position to try and get the number nine."
Following three appearances for Glasgow towards the end of last season, Cusiter travelled to Australia in order to get some more game time, featuring for Southern Districts in Sydney in three fixtures.
But he was still touch and go for selection after picking up a calf strain which hampered his training.
He added: "There was a lot of time in the summer when I still had this niggling injury which wouldn't go away and I was running out of time to prove my fitness.
"I probably had about two weeks of running and rugby under my belt before the Italy game, which was enough.
"I did miss a little bit at the start, but I did a lot of work with the fitness trainers to make sure I was still in good condition.
"Having been involved on Saturday and getting 20 minutes off the bench I feel great and really optimistic about what lies ahead."
Cusiter was one of three scrum-halves named in Scotland head coach Andy Robinson's 30-man squad on Monday, along with fellow British and Irish Lions Mike Blair and Rory Lawson.
All three have captained Scotland under the Englishman and the competition to start demonstrates the standard of the present Scotland squad.
Cusiter added: "The three of us have been rivals for six or seven years now.
"I don't know what's going to happen, but I'm confident in my ability. It's about proving ourselves in training and stating our case.
"Obviously only two guys are going to get in the squad out of three and one guy's going to start.
"I was on the bench in the last World Cup and I'd rather not be in that situation this time."
Cusiter's Glasgow team-mate Alastair Kellock was selected as captain of Scotland's World Cup squad.
But the Aberdonian, who was Robinson's first Test skipper and led the team for eight successive internationals, believes Blair and Lawson also have leadership credentials which will benefit Scotland.
He said: "It's been good for all of us. We're all probably some of the more experienced guys in the squad.
"We've had that experience of captaining our country.
"Hopefully we can help Al Kellock out whichever one of us plays."
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