Is there any good film on? 

=Is any good film on?

肯定句为：

There is any good film on.

=Any good film is on

请问：副词on可做补足语对吗？

副词可以作补足语：

下面4个句子都是副词做宾语补足语

(1)Please keep the dog out.

(2)Please let the fresh air in.

(3)Don't have the light on all the the time.

(4)An apple per day can keep a doctor away.

(5)Don't leave me alone.

解答如下：

以上为“副词作定语”

句子语法结构分析如下：

引导词：There

主语：any good film

谓语：is

后置定语：on(播放中的/上映中的/上演的)

Is there  any good film on？

有好的电影上映吗？

Is there anything on tonight? 

今晚有什么活动吗?
The people here/there were very friendly. 

那这里/那里的人很友好。
（备注：在通常情况下，副词用作定语总是放在被修饰的名词或代词之后。）

on（机器、电灯，电影等）开着，播放中，放映中，工作着，使用中

The light was on and the door was open.  

灯亮着，门也敞开着。

The central heating's been turned off. I've turned it on again.  

中央供暖系统已经关上。我又将它打开了。

Some guys are talking loudly while the film is on.  

电影开映的时候,还有些人在说话.

I hear a new film is on. How about seeing the film?  

听说正在上映一部新影片,去看看怎么样?

我同意 Webster 的看法。