As a spanish native speaker I can say I do understand a lot of words just by hearing them. Writing/reading is another thing tho, I can understand somewhat less. That doesn't mean I could speak it just like that tho. Some words are very similar and some are nothing alike. But this happens, for me, with italian and portuguese, you can make sense of what you hear pretty often. I couldn't say about italians understanding spanish, but my guess is it should be a similar experience. Hope this helps to answer your question. Cheers.

Of course being the native speaker of one of those languages will make it very easy to learn the other one. There are many similarities in grammar, vocabulary and sentence strucutre , the pronunciation remains quite different though.

El Español y el Italiano tienen raíces etimológicas comunes. Pertenecen a la misma familia idiomática: la de los idiomas latinos. Como tú dices son lenguas romances. Esa es la razón por la que muchas palabras del Italiano son similares y a veces idénticas a las correspondientes en Español. Pero son dos lenguas diferentes. El Italiano presenta una particularidad: tiene una diferencia entre lo escrito y lo hablado. Tiene un uso muy escaso del modo Subjuntivo en la expresión oral, lo que no sucede con el Español. Existe una similitud entre el Español y el Italiano en la estructura de las frases, en los artículos, sustantivos, adverbios, preposiciones y verbos, lo cuál hace presumir que quien habla una de las dos lenguas, entenderá facilmente la otra, pero eso no es así. Se necesita tener un conocimiento básico y un estudio de buén vocabulario por ambas partes.

I agree with Ivan83 and Robbie (I'm from Spain too). Both languages are not directly understandable, and have differences in entonation, rithm and vocabulary. Surely a child in a real encounter with an adult speaking the other language would have a hard time to catch the words. But there are many words with common origin, and so, very similar among them. So, after a while, one can get acustommed. If both speaker and listener have the will to understand, I think they can manage to maintain a conversation.

That's really cool....:P