No one else could have performed the nuances required for Dr. Lazarus.

Unironically true. While the catchphrase gets memed to shit all throughout the film the time he sincerely, and softly, speaks the whole thing as Quelik lies dying in his arms shifts the tone from pure humour to primal rage.

By Grabthar's hammer, by the Suns of Warvan, you shall be avenged.

Promised retribution aside Quelik dies happier than he ever might have expected because of that.

And then the unarmed, untrained civilian actor filled with fury charges and punches the alien soldier who had the gun in the face. [Yes the gun was jammed]