public class TestObject
{
    public string Name { get; set; }
    public int Age { get; set; }
}

<form action="/Create" method="post">
    <p>
        <%=Html.TextBox("Name") %>
    </p>
    <p>
        <%=Html.TextBox("Age")%>
    </p>
</form>

[AcceptVerbs(HttpVerbs.Get)]
public ActionResult Create()
{
    return View(new TestObject { Name = "DefaultName", Age = 10 } );
}

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Create(TestObject o)
{
    o.Name = "ChangedNameToSomethingElse";
    o.Age = 15;
    return View(o);
}