Tarkovsky is totally averse to symbolical or allegorical interepretations of his work. But there is a marvellous interview at Nostalghia.com where he speaks very candidly of Stalker.
Should I recall the plot of this extraordinary film? I tried to do so here. But to recall: Stalker leads two men he nicknames Writer and Professor through the Zone to reach the Room in which there is a golden ball which is said to grant the wishes of all who enter there. ‘Stalker’, we are to understand, is also a nickname: there was another Stalker, who was said to have been the greatest Stalker of them all, who was said to have entered the Room. He was the teacher of the present Stalker. One day, he returned from the Zone and found himself amazingly rich. Yet his brother had died in the Zone; the Master (as Stalker calls him) had led him to his death. The suggestion is that he found the golden ball and had had his wish granted: the death of his brother and countless riches. But then the Master hung himself; thereafter, he was known as Porcupine.
Who is Writer? A talented man who is burnt out. A popular writer who wants to enter the Room in order to recover his abilities, to find relief from the burden he is carrying. Later he worries that if he becomes a genius, he will stop writing, as everything he’ll write will be perfect. Writer then thinks about the story of the Master. Perhaps, he speculates, the Room grants you more than just the wishes you consciously select. Tarkovsky paraphrases Writer’s musings:
Perhaps they are true wishes pertaining to the inner world. If, let's say, I wish to become rich then I'll probably obtain not the riches but something more compatible with my nature, depth, the truth of my soul — for example poverty — which is closer to what my soul needs in fact.
What about Professor? He carries a bomb to blow up the Room since it is a place which will draw those who would wish for unlimited power and might endanger human life. Yet he gives up this plan – after all, those who set out for the Room desire only primitive things: money, fame, love. But he also comes to understand that the room offers something important: hope, longing, the ideal.
And what of Stalker? What is he looking for? Does he believe in the marvellous powers of the Room? Of the golden ball therein which will grant the wishes of the Writer and the Professor he has brought with him through the Zone? Tarkovsky:
Stalker does not enter the Room, that wouldn't be proper, that is not his role. It would be against his principles. Also, if all this is indeed a fruit of his imagination then he does not enter because he knows no wishes are going to be granted there. For him it is important that the other two believe in the Room's power and that they go inside. Stalker has a need to find people who believe in something in the world in which no one believes in anything.
Stalker is a man of faith, but faith in what? Perhaps he only has faith in faith – a faith that would retrieve faith from its disappearance from the world. Tarkovsky, speaking of Stalker:
He has a highly developed sense of his own worthlessness but at the same time he says to himself: why enter if nothing special happens there and most likely no wishes are granted? On the one hand he understands that wishes cannot be fulfilled and that they won't be fulfilled. And on the other, above all, he is afraid to enter. His approach is full of superstitions and contradictions. That's why Stalker is so depressed — nobody really believes in the Room's existence.
But it’s not the Room’s existence which concerns him as a wish-fulfilling device. Tarkovsky:
The Zone is in some sense a result of Stalker's imagination. Our line of reasoning was as follows: it is he who invented that place to bring people there and convince them about the truth of his creation [...] I completely agree with the suggestion that it was Stalker who had created the Zone's world in order to invent some sort of faith, a faith in that world's existence. It was a working hypothesis which we tried to preserve during creation of that world. We even planned an ending variant in which the viewer would find out Stalker had invented it all and now he is heartbroken because people do not believe him.
I have said that Stalker is a nickname – that there are other Stalkers. This is certainly the case in the novel on which the film is based. But Tarkovsky transformed that novel when he filmed it. Now, we are to be unsure whether the Room itself is not just Stalker’s invention.
Writer completely questions [the Room’s existence]. He says: "It probably doesn't exist" and he asks Professor: "Who told you this Room even existed?" The scientist points to Stalker. So he appears to be the sole witness. He is the only person who can testify to the existence of a Room with the power to grant wishes. He is the only one who believes. All the stories about the Room come from him — one could imagine he has invented it all.
I would like to reiterate something I said before. When he returns from the Zone, Stalker says to his wife of Professor and Writer: 'they are my friends'. Friends, it would seem, because they entertained a kind of hope with him. That they joined him, for a while in a kind of hope for hope: this was their friendship. And is it in the hope of a kind of friendship that Tarkovsky made this film? Where does he lead us, Tarkovsky the Stalker? To the brink of a Room we know is empty. Who is there? God? Nothing? But we remain with Tarkovsky at the brink - there where, just beyond, hopes and ideals once burned.
I like Stalker the most. The is the best part of myself and at the same time the least real one. Writer — who is very close to me — is a man who has lost his way. But I think he will be able to resolve his situation in the spiritual sense.