Former England boss Steve McClaren called time on his stint at the City Ground after just 111 days in the job two weeks ago and Cotterill has confessed that he was surprised when the role became available.
"I was very surprised [when McClaren resigned]," Cotterill told a press conference. "I didn't see that one coming at all.
"I certainly didn't expect to be sitting here now when Steve McClaren was unveiled during the summer."
The 47-year-old, who left Portsmouth last week, held a meeting with his new players this morning and revealed that he has acquired a squad that is low on confidence.
"We've got to improve our [league] position," he added. "I've spoken to the players this morning and they were a bit subdued.
"But by the time I left they were a bit more lively and hopefully that will continue tomorrow."
Forest are currently second from bottom in the Championship.