With huge Soviet and American flags draped behind them on the back wall, President Reagan and Soviet leader Mikhail S. Gorbachev chatted and gestured. They brimmed with confidence and a show of optimism Thursday as they watched top aides sign a series of minor agreements in the ceremony closing their summit in Geneva.

Another major event followed. Gorbachev returned to the Soviet Mission compound for a press conference, a rare event for a leader of the Soviet Union. Flanked by Kremlin aides, Gorbachev, looking at notes occasionally, presented his views of the summit in an hourlong monologue, followed by 40 minutes of questions. He spoke softly, gesturing often, sometimes cutting at his table with the side of his open hand.

Gorbachev acknowledged again that he and the President had failed to reach any understanding on the future of Reagan's "Star Wars" program. Their discussion, the Soviet leader said, had become "very lively indeed." While the President continued to describe his program as a defensive shield, Gorbachev continued to describe it as a potential space weapon that could destabilize the whole system of nuclear balance.