The stocky, redheaded Cosier was a middle-order batsman who often attacked the bowling when a more judicious method was the order of the day. Given an extended trial at international level over four seasons, he never really established himself as a Test batsman, although he was vice-captain of Australia for a brief period. He had two major highlights at Test level – a hundred on his Test debut at Melbourne against the West Indies in 1975–76, and a 168 against Pakistan the following season. Test bowlers were quick to exploit his technical deficiencies, in particular a very short backlift and abbreviated footwork. Cosier was all brute force with little finesse, but he did try to modify his technique somewhat during the 1978–79 Ashes series, when he was incongruously used as an opener.

He could back his batting with slow medium-pacers that swung alarmingly when the conditions favoured him. On the 1977 tour of England he showed in an ODI at Edgbaston that he could have been well suited to the burgeoning genre of one-day cricket when he snared five for 18. However, Cosier's bowling was not really used in Tests and his first Test wicket did not come until his 13th match. He was also an excellent close-in fielder and secure slipper.

Early career

Born and raised in Melbourne, Cosier attended University High School, where he captained the First XI and represented the Australian junior team in the West Indies. Highlights of that trip included 96 against Barbados.

Cosier played club cricket for Northcote where his captain was Bill Lawry. He made his first-class debut aged 18 for Victoria during the summer of 1971–72, opening with Lawry. Early sports writers sometimes compared him to Lawry. Cosier was a different style of batsman but later said "Bill's influence on me was in the way he could fight out of a situation. He was incredible the way he would bat with pain and with everything running against him."

During that summer he also appeared for the Victorian Colts side along with such players as Rodney Hogg and Paul Hibbert.

Cosier struggled with his weight throughout his career, in part due to his back trouble – a permanent condition since birth: a vertebrae is out of alignment. "I can't do any heavy physical work at all", he said in February 1977. "I can't run or do gym work."

Move to South Australia

He relocated to South Australia to play regular first class cricket. "I wasn't getting many runs, and one day keeper Ray Jordon told me to get across to Adelaide", he later recalled. "I thought no more of it until Ray, now a newspaper columnist, wrote a story saying I would definitely be going to South Australia". This resulted in a series of offers from Adelaide clubs which prompted Cosier to move in 1974.

Coming under the influence of South Australian skipper Ian Chappell, Cosier pursued his positive approach to batting and seemed to be headed for the top, although Chappell doubted his commitment to the game.

Highlights of the 1974–75 summer for Cosier included 81 against Victoria.

1975–76 Season

Cosier had an excellent season with the bat in 1975–76.

He leapt into test consideration with 130 for South Australia against the touring West Indians. He followed this up with 76 against NSW, and a match winning 49 against Queensland. He was named in a 13-person Australian squad to play a one-day international against the West Indies.

Cosier pressed his claims with 71 and 49 against Victoria although he injured his hamstring in the second innings at had to bat with a runner.

Cosier made his international debut for Australia against the West Indies in a one-day game. He took 1-33 and scored 25 not out, helping guide Australia to a five wicket victory.