Smiling in the face of a heretic, innovator or disbeliever

Smiling in the face of your Muslim brother is a required matter. As for a heretic, innovator or disbeliever, do not smile in his face unless it is to call him to Allaah, or fearing his evil, and the like. If your purpose is to call him to Allaah, and you smiled in his face, in order to accept your call, and for Allaah to guide him through you, you are upon a great good,

“Surely, if Allaah guides through you one man, this is better for you than red camels.” (Bukhaary 2942 and Muslim 2406)

Otherwise, the principle is,

Do not meet an innovator nor a heretic,            
without the frown of the angry Maalik!

Is such smiling and gentleness with the enemy leads to a benefit when calling to Allaah, there is no issue. As for a Muslim, it is not permissible to frown in his face. The Prophet ﷺ was indeed admonished in Allaah‘s  Words,

{عَبَسَ وَتَوَلَّىٰٓ ١ أَن جَآءَهُ ٱلْأَعْمَى}
“The Prophet frowned and turned away because there came to him the blind man, [interrupting].” (‘Abasa 1-2)

even though the Prophet ﷺ was preoccupied with calling others to Allaah, a matter he saw as more important than turning to the blind man. However, a Muslim still has rights and he has more right than others regarding respect and fulfilling such rights.