using System.Web.Mvc;
using EPiServer.Core;
using EPiServer.Framework.DataAnnotations;
using EPiServer.Framework.Web;
using EPiServer.Web;

namespace MySite
{
    [TemplateDescriptor(
        Inherited = true,
        TemplateTypeCategory = TemplateTypeCategories.MvcController,
        Tags = new [] { RenderingTags.Preview },
        AvailableWithoutTag = false)]
    public class PreviewController : Controller, IRenderTemplate<BlockData>
    {
        public ActionResult Index(IContent currentContent)
        {
            //TODO: Create model suitable for the view and return ViewResult
            return View();
        }
    }
}

Explained:

using System.Web.Mvc;
using EPiServer.Core;
using EPiServer.Framework.DataAnnotations;
using EPiServer.Framework.Web;
using EPiServer.Web;

namespace MySite
{
    [TemplateDescriptor(
        //Support everything inheriting from BlockData.
        Inherited = true,
        //By default controllers implementing IRenderTemplate<T> where T is BlockData
        //are registered as partial renderers. As this will be a "full page" renderer
        //we need to change that.
        TemplateTypeCategory = TemplateTypeCategories.MvcController,
        //Should only be used for preview
        Tags = new [] { RenderingTags.Preview },
        AvailableWithoutTag = false)]
    public class PreviewController : Controller,
        //Register as template for BlockData. To only support a specific type
        //change the type parameter from BlockData to that type and optionally
        //set Inherited = false in the the TemplateDescriptor attribute above.
        IRenderTemplate<BlockData>
    {
        public ActionResult Index(
            //While we implement IRenderTemplate for BlockData model binding
            //can only deal with IContent, ie shared blocks in this case.
            IContent currentContent
            )
        {
            //TODO: Create model suitable for the view and return ViewResult
            return View();
        }
    }
}