The phrase "smile a tremulant smile" may be a unique and interesting way to describe a smile that is somewhat shaky or quivery in nature. However, there are many other synonyms that could be used to describe a similar type of smile. For instance, a nervous or uncertain smile might be described as a hesitant smile or a self-conscious smile. A smile that is both shy and flickering could be referred to as a tentative grin or a fleeting smirk. Other synonyms for a tremulant smile might include a wavering smile, a quivering grin, a faltering smirk, or a hesitant smirk. Ultimately, the choice of words will depend on the individual's unique personal style and voice.