Although I can't tell you if your grandparents' surname is Lithuanian, I can tell you that immigrants were required to show official travel papers or passports before they were allowed to board the ship prior to sailing. The clerks would record the passengers' information on the manifest using the information on the travel papers or passports. So, whether or not a passenger was illiterate wouldn't matter. The information wasn't taken verbally but taken directly from the official identification each passenger had in his/her possession.
Two manifests were created prior to sailing: one to stay behind at the port of embarkation and one to sail with the ship for reference when the passengers debarked at the port of arrival.