Capricorn is an earth sign, stolid, steady and rooted. As a cardinal sign, it is a self-starter and a leader, but within institutional and organisational structures. Capricorn is the sign of empire building, but also the bureaucrat.
Someone born with Mercury in Capricorn is not likely to be capricious in thought or speech. Stephen Arroyo delineates Mercury’s placement in Capricorn as such:
Communicates seriously, cautiously, with a strong sense of authority; sometimes thinks in rigid categories Persistence, ambition, and steady progress satisfy one’s need to learn Self-controlled urge to express one’s perceptions and intelligence through manipulation of the physical world, and to get practical results from theories Reserved, self-sufficient, formal qualities can inhibit the way one communicates with others Reason and power of discrimination are used to build toward a goal; keen awareness of practical reality can lead to focus on limitations rather than possibilities (65)
People with Mercury in Capricorn include comedian Rowan Atkinson, feminist philosopher Simone de Beauvoir, mafia boss Al Capone, painter Paul Cézanne, actress Jane Fonda, feminist writer Germaine Greer, actor Paul Newman, and the ‘King’ himself Elvis Presley.
Arroyo, Stephen. Chart Interpretation Handbook. Sebastapol, Ca.: CRCS, 1989.