There is this one story in the Horus Heresy book Tales of Heresy, The Last Church by Graham McNeill, where the Emperor goes and wants to see the last church. Some quotes:
The Emperor enters church as appearing as a normal man:
A hooded figure was silhouetted in the doorway to the narthex, tall and swathed in a long cloak of scarlet.
'No,' said the man, pulling back his hood to reveal a stern, but not unkind face - a remarkably unremarkable face that seemed at odds with his martial bearing.
The chair creaked under the weight of his guest...
Now that Revelation had sat down, he had removed his scarlet cloak and draped it over the back of his chair. His guest wore utilitarian clothes, identical to those worn by virtually every inhabitant of Terra
The priest describes a vision he had:
'The power of the divine'. Said Uriah. 'I looked up and saw a golden face above me, a face of such radiance and perfection that my tears were no longer shed for pain, but for beauty.
The Emperor then reveals himself:
A golden light built behind Uriah and he saw his shadow thrown out onto the graven surface of the altar. The pearlescent hands of the clock shimmered in the reflected light and the ebony face gleamed.
Where once the church had been gloomy and filled with shadows, it was now a place of light.
Uriah pulled himself to his feet and turned to see a wondrous figure standing before him, towering and magnificent, clad in golden armour fashioned with love and the greatest skill, every plate embossed with thunderbolts and eagles.
Gone was Revelation, and in his place was a towering warrior of exquisite splendour, an exemplar of all that was regal and inspirational in humanity. The armour bulked his form out beyond measure and Uriah felt tears spilling from his eyes as he realised he had seen this breathtakingly, achingly perfect face once before.
So, the emperor appeared to be a normal man, with normal clothes in front of somebody who worships him but does not recognize him just to turn into a fully armoured god emperor (with towering size).