From Longman Dictionary of Contemporary Englishforce a smile/laugh etcforce a smile/laugh etcFORCE somebody TO DO somethingto make yourself smile, laugh etc even though you feel upset or annoyed → force
Examples from the Corpus
force a smile/laugh etc• He forces a smile, and Beulah smiles back.• I forced a smile and then turned and fled to my room.• The man turned out to be Alan Stewart, the comedian, who managed to force a smile as Roy apologised.• The girls forced smiles as they had been taught, but Sandi, in revenge, also crossed her eyes.• I forced a smile on seeing him. 