The פריערדיקער רבי writes in a letter: the main עבודה of ראש השנה is קבלת עול מלכות שמים. Therefore, the עבודה of the day involves saying תהלים the entire time, lessening one's sleep as much as possible on these two nights, and being extremely careful not to speak דברים בטלים. One needs to be like an עבד, who has no time to rest from the work of his master, or like a son who is joyfully occupied in greeting his father. (אגרות קודש מהוריי"ץ ח"י ע' תכ"ה)Once, on ראש השנה, the Tzemach Tzeddek's sons and some Chassidim went into the Rebbe and asked him to say חסידות during the time the עולם was saying תהלים. The Tzemach Tzeddek opened the door and said, "The עולם is saying תהלים, and you're saying "חסידות"?! It is better to say תהלים." And so they did.